Spark plug

ABSTRACT

The invention relates to a spark plug (1) comprising at least two insulated electrodes (5) and (6), an earthed electrode (7) and a chamber (4) so arranged with respect to one another that when a voltage is applied arcs are struck in series between electrodes (7) and (6) and (6) and (5) to provide a continuous plasma jet expelled from the chamber (4) at low energies (&lt;100 mJ) and which can be used for combustion of a lean air/fuel mixture.

TECHNICAL FIELD OF INVENTION

The invention relates to a spark plug, particularly though notexclusively for use in an internal combustion engine.

BACKGROUND ART

In common spark plugs, combustion is initiated from a plasma generatedby striking an arc between two electrodes. With typical spark energiesof 30 to 40 mJ. these devices are not capable of igniting mixtures withan air/fuel ratio significantly greater than stoichiometric.

Published work indicates that a jet of plasma is much more efficient atigniting mixtures with a high air/fuel ratio (see, for example, SAETechnical Papers 770355 and 800042). In spark plugs designed to generatejets of plasma at a high air/fuel ratio, a high energy (>1 Joule) isrequired to be dissipated in the arc.

DISCLOSURE OF INVENTION

It is an object of the invention to seek to mitigate this disadvantageof prior plasma jet type spark plugs.

A spark plug according to the invention is characterized by at least twoinsulated electrodes and an earth electrode so arranged with respect toone another that arcs are struck in series between successiveelectrodes.

Spark plugs embodying the invention are hereinafter described, by way ofexample, with reference to the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is schematic longitudinal sectional view of one proposed ortraditional spark plug;

FIG. 2 is a schematic longitudinal sectional view of another proposedspark plug;

FIG. 3 is a schematic longitudinal sectional view of a spark plugaccording to the invention;

FIG. 4 is a schematic longitudinal sectional view of a second embodimentof spark plug according to the invention;

FIGS. 5 to 8 are schematic longitudinal sectional views respectively ofdifferent electrode configurations of spark plugs according to theinvention;

FIG. 9 is schematic longitudinal sectional view of a further spark plugaccording to the invention; and

FIG. 10 is schematic lay-out of a driver unit for a spark plug accordingto the invention.

Referring to the drawings, FIG. 1 shows a spark plug in which combustionis initiated from a plasma generated by striking an arc betweenelectrodes A and B. With typical spark energies of 30 to 40 mJ. such aplug is not capable of igniting mixtures with an air/fuel ratiosignificantly greater than stoichiometric. FIG. 2 shows a cross sectionof a relevant part of another proposed spark plug which can generate aplasma jet for igniting combustible mixtures with a high air/fuel ratio.An arc is struck between end electrode A and centre insulated electrodeB, within cavity C in ceramic body D. Providing sufficient energy isdissipated in the arc (>1 Joule), a high level of ionization is producedin the cavity C. The energy dissipated also heats the gas, causing it toexpand rapidly. Consequently, ionized gas is ejected from the cavity Cas a plasma jet E.

Spark plugs embodying the invention are illustrated in FIGS. 3-9, inwhich like reference symbols are used for like parts.

Referring to FIG. 3 there is shown a spark plug 1 embodying theinvention, which is essentially a twin gap spark plug for producing aplasma jet at E. The relevant, spark forming part of the spark plug 1has an insulator in the form of a ceramic body 2 terminating at asurface 3, the insulator encompassing a cavity 4 in which there is a(central) insulated electrode 5. There is also an insulated electrode 6and an earth electrode 7. The electrodes are spaced apart and formsequential pairs 7-6 and 6-5. More particularly, as will be appreciatedfrom the drawing, electrode 5 is situated at the base of the cavityinterior while electrode 6 is situated to one side of the cavity, with aportion of the electrode 6 adjacent the cavity orifice or exit. Groundelectrode 7 is spaced across the cavity exit from electrode 6, as shown.Thus, electrodes 5 and 6 define a first spark gap across a portion ofthe cavity interior and electrodes 6 and 7 define a second spark gapacross the cavity exit.

The spark plug 1 embodying the invention operates as follows:

Arcs are struck simultaneously between electrodes 7 and 6, and 6 and 5,the arcs being in series. The heat generated by the arc betweenelectrodes 6 and 5 causes the gas in the cavity 4 to expand and beejected from the cavity 4. As the expanding gas escapes from the cavity4 it has to pass through the arc between the electrodes 7 and 6, and isin consequence ionized. Hence a jet of plasma is generated at E.

This system of plasma jet generation is very efficient and will workwith spark energies of <100 mJ, for example in the range of about 50 mJto 100 mJ.

The operation of the device is self-stabilizing as a consequence ofnegative feedback inherent in the design. The arcs 7-6, 6-5 are inseries. Any tendency for the arc 7-6 to be extinguished by the expandinggas passing through it will reduce the arc current. This will alsoreduce the current through arc 6-5, hence reducing the heat dissipationwithin cavity 4. The gas will therefore expand more slowly, reducing thetendency to extinguish arc 7-6. Thus two spark gaps in series are used.One spark gap is inside the cavity to heat and expand the gas. The otheris across the orifice of the cavity to ionize the expanding gas as it isejected from the cavity. It is possible to generate a plasma jet withless than 100 mJ of energy.

Referring now to FIG. 4, there is shown a longitudinal sectional view ofa part of another spark plug 1, according to the invention, which cangenerate a plasma jet for igniting combustible mixtures with a highair/fuel ratio. The relevant, spark forming part of the spark plug 1 hasan insulator in the form of a cermaic body 2 terminating at a surface 3,the insulator encompassing a cavity 4 in which there is a (central)insulated electrode 5. There is also an insulated electrode 6 and anearth electrode 7.

The electrodes 6 and 7 are each extended upwardly, as viewed, above oraway from the surface 3 of the body 2. In the embodiment shown, therespective extensions 8 and 9 of the electrodes 6 and 7 diverge upwardlyas viewed.

In use, arcs are struck simultaneously between electrodes 7 and 6, and 6and 5 which arcs are in series. The gas expanding out of the cavity 4produces a sheet kind of discharge as indicated at 10. This sheetdischarge effect cannot be obtained by striking an arc between theelectrodes 7 and 6 only, which would merely provide a normal narrowspark discharge. The spark plug 1 according to the invention thusproduces a sheet discharge which provides for ionization of the hot gasexpanding in and being ejected from the cavity 4 so producing a jet ofplasma as indicated by arrow `E`. The jet E then provides reliablecombustion of a lean air/fuel mixture.

Because extensions 8 and 9 of the electrodes provide a sheet discharge,the spark plug gap has in effect been extended, but the voltage does nothave to be increased to achieve combustion. Further, the spark forionizing the gas is self-stabilizing because any tendency for the sparkbetween electrodes 6 and 7 to be extinguished by the gas expanding outof the cavity 4 will tend to reduce the arc current, hence reducing theheat dissipation in the cavity 4. This will result in the gas expandingmore slowly, reducing the tendency to extinguish the spark betweenelectrodes 6 and 7 and so maintaining the sheet discharge 10 and fullionization of the gas. The net result is that the plasma jet E ismaintained.

The plasma jet E can be produced with less than 100 mJ of energy in sucha plug.

It will be understood that the spark plug shown in the drawing and abovedescribed may be modified. For example, the parts 8 and 9 may convergeor may be substantially parallel.

Referring now to FIGS. 5 to 8, different electrode configurations ofspark plugs according to the invention are illustrated. In FIG. 5,electrodes 6 and 7 are flush with the surface 3 while in FIG. 6 there isshown electrodes 6 and 7 which extend outwardly of the surface 3. FIG. 7shows electrodes in which the surface 3 itself is inclined to provide agenerally frusto-conical configuration, the electrodes 6 and 7 beingshaped to provide inclined electrodes which protrude above the surface 3as in FIG. 4.

The motion of the jet in use in this embodiment draws fresh charge intothe active region adjacent the plug thus encouraging mixing andimproving combustion.

Similarly in FIG. 8, the electrodes 6 and 7 have extensions 8 and 9respectively, the electrodes being laid on the surface of the insulatorrather than being buried in it in order to maximize exposure of the fuelcharge to the arc.

In FIGS. 5 to 8, there is shown a metal body 11 of the plug 1 with partof a screw thread by which the plug 1 may be screwed into an internalcombustion engine. There is a central rod electrode 5 which is sheathedin a plastics material 12 such as polytetrafluorethylene (PTFE) excepton the face exposed to the interior of the cavity 4.

FIG. 9 shows another embodiment of spark plug electrode 6 comprising twoparts 6A and 6B which are linked electrically as shown. The series arcsbetween electrodes 7-6A and 6B-5 are shown, there being provided a sheetdischarge between extensions 8 and 9 in use.

In FIGS. 5 to 9, the exit from the cavity 4 is narrower than the cavityitself. In FIG. 9 the exit may be 1 mm in width, the volume of thecavity 4 being 28 mm³, the extensions being 3 mm "long" (i.e., from thefree ends of the extensions to the surface 3, measured vertically asviewed).

In the embodiments shown in FIGS. 4, 8 and 9, for example, theextensions 8 and 9 may be formed by wires secured to the electrodes 6,7,by a tube secured to those electrodes, or by integrally forming theelectrodes to form the required shape of extension(s). These extensionsmodify the shape of the arc/plasma jet to provide a continuouscombustion of lean mixtures. The net result stated another way, in eachembodiment a stable sheet discharge is formed which can be generated atlow energies and which can be used for the ignition of lean mixtures offuel in internal combustion engines.

FIG. 10 shows schematically a driver unit which can be used for drivinga spark plug 1 embodying the invention.

The plug 1 is connected in a circuit including a battery 12 (+350 Vsupply), an electronic switch 13, a capacitor 14 and ignition coil 15.

The capacitor may have a capacitance of 1 μF.

The stored energy is then:

    (CV.sup.2)/2=60 mJ

Using such a circuit with the switch closed for 0.5 m seconds, a peakspark current of about 75 mA falling exponentially to 20 mA peak at theend of the 0.5 ms period is achieved, the circuit "ringing" at about 4KHZ.

The volume of the cavity 4 may be varied by making the position of theelectrode 5 relative to the body 2 adjustable.

The space between facing surfaces of electrodes 6 and 7 may have awidth/diameter of 0.508 mm which is also equivalent to the lateralextent of the cavity. The distance between the base (as viewed) of theelectrode 6 and the top (as viewed) of the electrode 5 may be 0.381 mm.

It will be understood that a spark plug embodying the invention has manyapplications, and is not just for internal combustion engines, forexample:

1. Other types of engine where the plug could be used are:

(a) Gas Turbines

(b) Gas Engines

(c) Spark Arrested Diesels

(d) Any other type of engine requiring or benefitting from a spark toinitiate combustion.

2. The plug could be used as an ignitor in static applications:

(a) Boilers

(b) Heaters

(c) Process Plant

(d) Gas Jets

(e) Blow Lamps

(f) Torches.

3. The plug could also be used anywhere a pulsed source of ions issuitable:

(a) Cleaning by Ion Bombardment

(b) Surface Treatment

(c) Inducing Chemical Changes in Surfaces

(d) Triggering Electrical Discharges

(e) Displays

(f) Inducing Chemical Reactions.

In every embodiment, the use of two arcs in series utilizes theavailable energy more efficiently, a larger proportion thereof beingdissipated in the arcs and less in the (external) driving circuitry.

I claim:
 1. A spark plug including means defining a spark cavity havinga base, sidewalls, and an exit opening providing a path of escape fromthe cavity for gases expanding therein,first electrode means defining afirst spark gap across an interior portion of said cavity, and secondelectrode means defining a second spark gap extending across said exitopening, said second electrode means being positioned relative to saidfirst electrode means such that gases escaping from the cavity interioralong said path of escape as a result of a spark across said first sparkgap must traverse said second spark gap, said second electrode meansfurther being in series with said first electrode means, whereby anytendency of said escaping gases to reduce spark current at said secondspark gap causes a similar spark current reduction at said first sparkgap, thereby reducing the flow of escaping gases and stabilizing thespark currents at said first and second spark gaps.
 2. A spark plugaccording to claim 1, wherein said first electrode means comprises apair of first electrode members including an insulated electrode membersituated proximate the base of said cavity and an insulated electrodemember situated proximate a sidewall of said cavity, and wherein saidsecond electrode means comprises a pair of second electrode membersincluding an insulated electrode member positioned to one side of saidexit opening and an additional electrode member spaced across said exitopening from that insulated electrode member.
 3. A spark plug accordingto claim 2, wherein said second electrode members are flush with asurface of the plug.
 4. A spark plug according to claim 2, wherein saidsecond electrode members extend outwardly of a surface of the plug.
 5. Aspark plug according to claim 2, wherein said second electrode membershave respective extensions projecting outwardly of a surface of theplug.
 6. A spark plug according to claim 2, wherein said electrodemember positioned proximate a sidewall of said cavity and said electrodemember positioned to one side of said exit constitute portions of asingle insulated electrode.
 7. A spark plug according to claim 5,wherein said extensions comprise wires secured to the respective secondelectrode members.
 8. A spark plug according to claim 5, wherein saidextensions comprise tubular extension means secured to the respectivesecond electrode members.
 9. A spark plug according to claim 2, whereinthe volume of the cavity is adjustable.
 10. A spark plug according toclaim 2, wherein said exit of said cavity is narrower in width than theinterior of said cavity.
 11. A spark plug according to any of claims 5to 8, wherein said extensions have respective free ends spaced furtherapart than ends of said extensions adjacent the surface of the plug. 12.A spark plug according to any of claims 3 to 8, wherein said surface isof generally frusto-conical configuration.